Desulfurization composition

ABSTRACT

A desulfurization composition includes calcium oxide, silicon dioxide and aluminum oxide. The desulfurization composition is free of an alkali metal oxide. Aluminum oxide is present in an amount ranging from 20 to 26 wt % based on the total weight of the desulfurization composition, and the weight ratio of calcium oxide to aluminum oxide is within a range of 2.19 to 3. A method for desulfurizing molten steel using the desulfurization composition is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Patent Application No.104103462, filed on Feb. 2, 2015, the entire disclosure of which ishereby incorporated by reference.

FIELD

The disclosure relates to a desulfurization composition, moreparticularly to a desulfurization composition for desulfurizing moltensteel.

BACKGROUND

During the manufacture of steel in a steel mill, sulfur impurities maybe removed via a desulfurization process so as to improve steel quality.Specifically, a desulfurization agent with a low melting point and lowviscosity is added to molten steel. The temperature and the partialpressure of oxygen during this desulfurization process are typically setat about 1600° C. and under 20 ppm, respectively.

A conventional method used to increase the desulfurization efficiency ofthe desulfurization agent and to improve steel quality involves theaddition of quicklime (calcium oxide, CaO). As the amount of addedquicklime increases, the viscosity of the desulfurization agent may alsoincrease to a point that impedes the desulfurization process. To counterthis effect, calcium fluoride (CaF₂) may be added to decrease theviscosity of the desulfurization agent. However, calcium fluoride mayerode a furnace lining of a furnace used in the manufacture of steel,and the fluorine evaporated during the desulfurization process wouldgreatly harm the human body and environment.

CN 103882183 A discloses a calcium fluoride-free desulfurizationrefining slag containing 50-60 wt % of calcium oxide, 7-12 wt % ofsilicon dioxide (SiO₂), 28-33 wt % of aluminum oxide (Al₂O₃) and 4-8 wt% of magnesium oxide (MgO). The weight ratio of calcium oxide toaluminum oxide ranges from about 1.52 to 2.14, preferably from about 1.7to 1.9. After desulfurization, the melted refining slag mainly contains12CaO.Al₂O₃. However, the desulfurization efficiency of thedesulfurization refining slag disclosed therein is quite low.

Therefore, the applicants have endeavored to find a calciumfluoride-free desulfurization composition that is associated withimproved desulfurization efficiency.

SUMMARY

Therefore, a first object of the disclosure is to provide adesulfurization composition that can alleviate at least one of thedrawbacks of the prior art.

According to the disclosure, the desulfurization composition includescalcium oxide, silicon dioxide and aluminum oxide. The desulfurizationcomposition is free of an alkali metal oxide. Aluminum oxide is presentin an amount ranging from 20 to 26 wt % based on the total weight of thedesulfurization composition, and the weight ratio of calcium oxide toaluminum oxide is within a range of 2.19 to 3.

A second object of the disclosure is to provide a method fordesulfurizing molten steel comprising mixing the aforesaiddesulfurization composition with molten steel.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparent inthe following detailed description of the embodiments with reference tothe accompanying drawings, of which:

FIG. 1 is an x-ray diffraction pattern of a desulfurization compositionof Example 1 that is heated to 1600° C. and then cooled to below 30° C.

FIG. 2 shows a differential thermal analysis (DTA) curve of thedesulfurization composition of Example 1;

FIG. 3 shows a DTA curve of a desulfurization composition of ComparativeExample 1;

FIG. 4 shows the viscosity of the desulfurization composition of Example1 at different temperatures;

FIG. 5 shows the viscosity of the desulfurization composition ofComparative Example 1 at different temperatures;

FIG. 6 shows the outer appearance of a magnesia carbon brick afterimmersion in the melted desulfurization composition of Example 1;

FIG. 7 shows an end surface of the magnesia carbon brick shown in FIG.6;

FIG. 8 shows the outer appearance of a magnesia carbon brick afterimmersion in the melted desulfurization composition of ComparativeExample 2;

FIG. 9 shows an end surface of the magnesia carbon brick shown in FIG.8;

FIG. 10 shows the outer appearance of a magnesia carbon brick afterimmersion in the melted desulfurization composition of ComparativeExample 5; and

FIG. 11 shows an end surface of the magnesia carbon brick shown in FIG.10.

DETAILED DESCRIPTION

An embodiment of a desulfurization composition of this disclosureincludes calcium oxide, silicon dioxide and aluminum oxide and is freeof an alkali metal oxide. Based on the total weight of thedesulfurization composition, aluminum oxide is present in an amountranging from 20 to 26 wt %. The weight ratio of calcium oxide toaluminum oxide is within a range of 2.19 to 3.

In certain embodiments, the weight ratio of calcium oxide to aluminumoxide is within a range of 2.2 to 2.5.

In certain embodiments, aluminum oxide is present in an amount rangingfrom 24 to 26 wt %.

In certain embodiments, calcium oxide is present in an amount rangingfrom 57 to 60 wt % based on the total weight of the desulfurizationcomposition. In certain embodiments, calcium oxide is present in anamount ranging from 57 to 59 wt %.

In certain embodiments, silicon dioxide is present in an amount rangingfrom 7 to 17 wt % based on the total weight of the desulfurizationcomposition. In certain embodiments, silicon dioxide is present in anamount ranging from 10 to 14 wt %.

The desulfurization composition of the disclosure may further includemagnesium oxide. Magnesium oxide may be present in an amount rangingfrom 1 to 10 wt % based on the total weight of the desulfurizationcomposition. In certain embodiments, magnesium oxide is present in anamount ranging from 3 to 7 wt %.

The desulfurization composition of the disclosure may further includeother components, including but not limited to dolomite [CaMg(CO₃)₂] orbauxite (aluminous soil).

The desulfurization composition of this disclosure can be mixed withmolten steel under heating (at, e.g., 1600° C.) to performdesulfurization of molten steel. The desulfurization composition afterheating includes Ca₃SiO₅ and 3CaO.Al₂O₃. Since Ca₃SiO₅ and 3CaO.Al₂O₃have relatively low melting points and similar viscosities to that ofCaF₂, the desulfurization composition of the disclosure exhibits betterdesulfurization efficiency for molten steel.

The disclosure will be further described by way of the followingexamples. However, it should be understood that the following examplesare solely intended for the purpose of illustration and should not beconstrued as limiting the disclosure in practice.

EXAMPLES Preparation of the Desulfurization Composition Example 1 (E1)

Calcium oxide (Cao, 95% purity), aluminum oxide, silicon dioxide andmagnesium oxide powders were mixed to form a desulfurizationcomposition. The amounts of the aforesaid components based on the totalweight of the desulfurization composition are listed in Table 1. Itshould be noted that calcium oxide may be obtained by heating calciumcarbonate, and thus calcium oxide having a purity of 95% can be replacedby calcium carbonate having a purity of 98%.

TABLE 1 Amount (wt %) CaO/Al₂O₃ CaO Al₂O₃ SiO₂ MgO (wt %/wt %) E1 58 2512 5 2.32

Comparative Examples 1 to 5 (CE1-5)

Each of the desulfurization compositions of Comparative Examples 1 to 5was prepared by mixing components at particular weight percentages asshown in Table 2.

TABLE 2 Amount (wt %) CaO/Al₂O₃ CaO Al₂O₃ SiO₂ MgO CaF₂ Na₂O (wt %/wt %)CE1 52 28 5 5 10 — 1.86 CE2 60 20 12 5 5 — 3 CE3 50 15 30 5 — — 3.33 CE450 35 15 0 — — 1.43 CE5 60 20 10 5 — 5 3 —: not added

X-ray Diffraction (XRD) Analysis

The desulfurization composition of Example 1 was placed in a graphitecrucible and was heated to 1600° C. under 1 atm in a frequency furnaceto evenly melt the desulfurization composition. The melteddesulfurization composition was poured out from the graphite crucible,and was cooled to a temperature below 30° C. so as to obtain a treateddesulfurization composition. The treated desulfurization composition wassubjected to XRD analysis. The XRD pattern is shown in FIG. 1. FIG. 1indicates that the treated desulfurization composition mainly includesCa₃SiO₅ and 3CaO.Al₂O₃.

Differential Thermal Analysis (DTA)

The desulfurization compositions of Example 1 and Comparative Example 1were subjected to differential thermal analysis using Linseis STA PT1600. The heating and cooling rate was 2° C./min, and the results areshown in FIG. 2 (Example 1)and FIG. 3 (Comparative Example 1).

As shown in FIGS. 2 and 3, the melting point of the desulfurizationcomposition of Example 1 is 1493° C., which is less than the meltingpoint (1535° C.) of Comparative Example 1. These results demonstratethat a low melting point of the desulfurization composition can beobtained by adjusting the weight percentages of the components thereofwithout the addition of calcium fluoride.

Viscosity Analysis

The viscosities of the desulfurization compositions of Example 1 andComparative Example 1 were measured by a viscometer(Brookfield-DV DIRV). The results are shown in FIG. 4 (Example 1) and FIG. 5 (ComparativeExample 1).

As shown in FIGS. 4 and 5, the viscosities of the desulfurizationcomposition of Example 1 at 1600° C. and 1550° C. (116 cP and 122 cPrespectively) were less than those of Comparative Example 1 (122 cP and129 cP). These results demonstrate that a low viscosity of thedesulfurization composition can be obtained by adjusting the weightpercentages of the components thereof without the addition of calciumfluoride.

Desulfurization Rate Analysis

Thermo-Calc software, coupled with TCS Steels/Fe-Alloys Database v7.0and TCS Fe-containing Slag Database v3.2, was used to determine thedesulfurization rate of the desulfurization compositions of Example 1and Comparative Examples 1 to 3. The parameters listed in Table 3 wereinputted into the software. The amount of the desulfurizationcomposition was set to be 2 wt % based on the total weight of a mixturecontaining the desulfurization composition and molten steel.

TABLE 3 Inputted parameter Main component of molten Iron steel Initialamounts of carbon Carbon: 0.05 and sulfur in molten steel Sulfur: 0.025(wt %) Initial amounts of Ca, Si, Initial amounts of Ca, Si, Al and Mgin the Al and Mg in the desulfurization composition were desulfurizationcomposition determined by the components of each of (wt %) thedesulfurization compositions of Example 1 and Comparative Examples 1 to3 Initial amount of oxygen in Initial amount of oxygen in the mixturewas the mixture (wt %) determined by the components of each of thedesulfurization compositions of Example 1 and Comparative Examples 1 to3 Temperature 1580~1620° C.

The graph obtained from the software shows the amounts of C, S, O, Ca,Si, Al and Mg included in the mixture after desulfurization. The amountof sulfur (wt %) at 1600° C. was used to calculate the desulfurizationrate. The amount of oxygen (wt %) at 1600° C. was used to calculate apartial pressure (ppm) of oxygen by unit conversion.

The desulfurization rate of the desulfurization composition wascalculated according to the following equation:

A=(B−C/B)×100%

-   -   where A=desulfurization rate (%)        -   B=initial amount of sulfur in molten steel(wt %)        -   C=the amount of sulfur in molten steel (wt %) at 1600° C.

<Results of Desulfurization Rate Analysis>

It should be noted that the partial pressure of oxygen is an importantfactor for the desulfurization rate. In the desulfurization process, thelower the partial pressure of oxygen is, the higher the desulfurizationrate will be (i.e., the better the desulfurization effect will be).

a) Comparison between Example 1 and Comparative Example 1:

The amounts of sulfur after desulfurization and the desulfurizationrates of Example 1 and Comparative Example 1 are listed in Table 4.

TABLE 4 CaO/ Partial Amounts of Desul- Amount Al₂O₃ pressure sulfurafter furiza- (wt %) (wt %/ of oxygen desulfurization tion rate CaF₂Al₂O₃ wt %) (ppm) (wt %) (%) E1 0 25 2.32 10.3 0.009 64 CE1 10 28 1.8611 0.0083 66.8

As shown in Table 4, the desulfurization rates of Example 1 andComparative Example 1 are quite close under conditions in which thetemperature is 1600° C. and partial pressure of oxygen of the twoexamples are similar.

The results demonstrate that, without calcium fluoride, thedesulfurization composition containing the components with particular

weight percentages still exhibits a high desulfurization rate.

(b) Comparison between Example 1 and Comparative Examples 3 and 4:

The amounts of sulfur after desulfurization and desulfurization rates ofthe desulfurization compositions of Example 1 and Comparative Examples 3and 4 are listed in Table 5.

TABLE 5 Amounts of CaO/ Partial sulfur Al₂O₃ pressure of afterDesulfurization Al₂O₃ (wt %/ oxygen desulfurization rate (wt %) wt %)(ppm) (wt %) (%) E1 25 2.32 10.3 0.009 64.0 8.8 0.00786 68.6 3.770.00408 83.7 2.53 0.00272 89.1 2.28 0.00213 91.5 1.74 0.0016 93.6 CE3 153.33 9.7 0.0188 24.8 4.2 0.0123 50.8 2 0.0104 58.4 CE4 35 1.43 9.7 0.01444 4.97 0.0091 63.6 2.6 0.0047 81.2

As shown in Comparative Example 3 of Table 5, in which the amount ofaluminum oxide was less than 20 wt % and the weight ratio of calciumoxide to aluminum oxide was greater than 3, the desulfurization rate was58.4 wt % when the partial pressure of oxygen was 2 ppm. Thedesulfurization rate in Comparative Example 3 was less than thedesulfurization rate of Example 1 (83.7 wt %) when the partial pressureof oxygen was 3.77 ppm. Furthermore, in Comparative Example 4 in whichthe amount of aluminum oxide was greater than 26wt % and the weightratio of calcium oxide to aluminum oxide was less than 2.19, thedesulfurization rate was 81.2 wt % when the partial pressure of oxygenwas 2.6 ppm. The desulfurization composition of Example 1 exhibits abetter desulfurization rate under a relatively high partial pressure ofoxygen (3.77 ppm). It can be predicted that the desulfurizationcomposition of the disclosure would have a higher desulfurization ratethan that of Comparative Examples at the same partial pressure ofoxygen. The results reveal that the desulfurization rate can beeffectively enhanced depending on the particular weight percentages ofthe components within the desulfurization composition of the disclosure.

Erosion Rate Analysis

It should be noted that the manufacture of steel is conducted at arelatively high temperature (e.g., 1600° C.), and a furnace lining of ahigh temperature furnace used in the manufacture of steel has to be madeof a refractory material. The commonly used lining material is magnesiacarbon brick. The following experiment was performed to evaluate theeffect of the desulfurization compositions of Example 1 and ComparativeExamples 2 and 5 on the erosion rate of the magnesia carbon brick.

Each of the desulfurization compositions of Example 1, ComparativeExample 2 and Comparative Example 5 was placed in a graphite crucible,and then heated to 1600° C. in a frequency furnace until melted. Asquare columnar magnesia carbon brick was then immersed in a respectiveone of the resultant melted desulfurization compositions for sixtyminutes. Before and after immersion, a projected area of an end surfaceof the magnesia carbon brick on a plane perpendicular to an axis of thecolumnar magnesia carbon brick was calculated by multiplying a width anda length of the end surface. Erosion rate (%) was calculated accordingto the following equation:

D=(E−F/E)×100%

-   -   where D=erosion rate        -   E=projected area of the end surface of the magnesia carbon            bricks before immersion in the melted desulfurization            composition        -   F=projected area of the end surface of the magnesia carbon            bricks after immersion in the melted desulfurization            composition for 60 minutes

The length, width and projected area of the end surface of the magnesiacarbon brick before and after immersion as well as the calculatederosion rate are summarized in Table 7. It should be noted that a lowererosion rate indicates that the desulfurization composition causes lesserosion to the magnesia carbon brick. The outer appearances of themagnesia carbon bricks after immersion in the melted desulfurizationcompositions of Example 1, Comparative Example 2 and Comparative Example5 are respectively shown in FIGS. 6-7, 8-9, and 10-11. The white portioncovering a peripheral region of the magnesia carbon brick is thedesulfurization composition.

TABLE 6 Magnesia carbon brick Length Width Projected Erosion Immersion(mm) (mm) area (mm²) rate (%) E1 before 14.47 13.36 193.32 4.6 (w/o Na₂Oafter 14.10 13.08 184.43 and CaF₂) CE2 before 15.12 12.86 194.44 6.5(with CaF₂) after 14.54 12.50 181.75 CE5 before 15.53 14.42 223.94 6.6(with Na₂O) after 15.22 13.74 209.12

As shown in FIGS. 6-11, the magnesia carbon brick immersed in the melteddesulfurization composition with CaF₂ (Comparative Example 2) or Na₂O(Comparative Example 5) is more seriously eroded as compared to themagnesia carbon brick immersed in the desulfurization compositionwithout Na₂O and CaF₂ (Example 1). Table 6 also reveals that thedesulfurization compositions of Comparative Examples 2 and 5 exhibithigher erosion rates than that of Example 1. These data demonstrate thatthe desulfurization composition of this disclosure exhibits less erosiveeffects on the magnesia carbon brick (lining material), thus extendingthe service life of the high temperature furnace used for themanufacture of steel and increasing the use frequency of the furnace.

To sum up, by adjusting the weight percentage of aluminum oxide and theweight ratio of calcium oxide to aluminum oxide, the desulfurizationcomposition according to this disclosure could obtain a desired meltingpoint and viscosity, and exhibit higher desulfurization efficiency andless erosion rate to the furnace lining.

While the disclosure has been described in connection with what areconsidered the exemplary embodiments, it is understood that thisdisclosure is not limited to the disclosed embodiments but is intendedto cover various arrangements included within the spirit and scope ofthe broadest interpretation so as to encompass all such modificationsand equivalent arrangements.

What is claimed is:
 1. A desulfurization composition comprising calciumoxide, silicon dioxide and aluminum oxide, wherein said desulfurizationcomposition is free of an alkali metal oxide, and wherein aluminum oxideis present in an amount ranging from 20 to 26 wt % based on the totalweight of said desulfurization composition, and the weight ratio ofcalcium oxide to aluminum oxide is within a range of 2.19 to
 3. 2. Thedesulfurization composition as claimed in claim 1, wherein calcium oxideis present in an amount ranging from 57 to 60 wt % based on the totalweight of said desulfurization composition.
 3. The desulfurizationcomposition as claimed in claim 1, wherein silicon dioxide is present inan amount ranging from 7 to 17 wt % based on the total weight of saiddesulfurization composition.
 4. The desulfurization composition asclaimed in claim 1, further comprising magnesium oxide.
 5. Thedesulfurization composition as claimed in claim 4, wherein magnesiumoxide is present in an amount ranging from 1 to 10 wt % based on thetotal weight of said desulfurization composition.
 6. A method fordesulfurizing molten steel comprising mixing the desulfurizationcomposition as claimed in claim 1 with molten steel.